Selmer Constellation 20...............crocodile rock
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This page was last modified on Saturday, February 04, 2012
1) Dating from about 1965, this is the second generation of Constellations, uprated to 20 watts and fitted with 2 x 12" speakers, in this case R&As. It has two channels with two inputs each, volume controls for each channel, shared treble and bass plus speed and depth controls for the tremolo. It's had a major burn up at some time and been stuck in someone's shed for years. That's a real shame, because it's not in bad condition externally and it'll make a really nice rehearsal / recording amp when it's up and running again.
This one is beyond economic restoration to original condition, but it'll make a very good renovation project.
2) First sign of trouble is the large hole in the paxolin circuit board. It's beyond repair so I've made a new one out of fibreglass. The track layout and heavy solder tinning is exactly the same as the original apart from the area around the electrolytics. Resistors and electrolytics are brand new while the bias diode and Wima caps are original.
3) R&A Loudspeakers:
Selmer seem to have used several speaker suppliers. John Chambers at Champ Electronics has some pictures of a Constellation fitted with RTCs, http://www.chambonino.com/work/selmer/selm5.html and Steve Russel and Tim Fletcher have a Celestion G12 version at http://www.vintagehofner.co.uk/.
These two were made by R&A, 'Reproducers And Amplifiers Ltd' of Frederick Street, Wolverhampton. They were known for supplying good quality speakers to major radio and radiogram manufacturers, hence 'Higher Fidelity'. I believe the Company was dissolved in 1973. They had no connection with Richard Allan despite the initials.
I haven't tried these yet, but the coils are intact and don't rub and they appear to be original to the cabinet. At a guess, I'd say that they're a version of the model 7120 without the fancy chrome plating and with a bell cover to make them look more impressive. If that's the case, they're 15ohm units rated at 10watts with an LF resonance of 50Hz. In 1965 the standard versions would have cost £4 18s 6d each.
The bell cover bolt has sheared off in the magnet on the left and the rust bug has got to both frames. It's not terminal though and I've test sprayed the loose cover with red oxide and CarPlan 'Smoke Blue' touch up paint. The match is almost perfect but, sod's law, I don't think they make it any more. Oh well, try again. When I get really bored I'll mask off the cones and refinish both drivers. I can see a possible problem with keeping the metal dust off the magnets during the rubbing down part.
4) Schematic
This is a CAD version of the old Selmer drawing No.2372.....
.........and the corresponding main board layout.
If you compare the track layout with the original circuit board you can see that V5 has had a bit of a bad day and taken out the screen grid resistor, the track and the board. Probably couldn't face playing Yellow River any more.
I should think the amount of damage was down to the home made fuse. The original 1amp Belling had already vapourised.
There's another problem on the actual front panel circuit board. The track that joins C8 and C9 to the wiper of the bass pot is missing. There's no sign of it ever having been there so I guess they're all the same.
It'll be interesting to hear the difference with and without when it's back together.
5) This is the front panel pcb as found.
6) On the way to recovery.
7) More fireworks.
How bad can this thing get? This is where the mains switch has arced across to the chassis and blown a lump out of it. The switch, although it doesn't fit, is original and gives some idea of how dangerous old amps can be.
It gets worse. Someone has disconnected the mains earth wire and carefully taped it back. Thinking about it, the earth must have been connected when the switch shorted for the first time or there wouldn't have been any current draw. Now though the whole thing is a death trap.
Have a look at these pictures and then check out the Watkins Copicat section for a discussion about Portable Appliance (PAT) Testing.
Both sides of the mains switch measured open circuit on a multimeter, but that's not unusual for an old switch that hasn't been used for years. The problem is internel dirt and corrosion, particularly in this type with rollers instead of wiping contacts. Quite often the mains voltage will punch through the corrosion although arcing will eventually destroy the switch.
The internals of this one could be cleaned up, but the plastic moulding has been charred when it shorted to the chassis. The flash has also deposited a thin film of metal right across the terminal area. Exit one switch.
8) Pot Troubles
Bit of a problem finding replacement pots. The front panel is spaced away from the pcb by an extra pot nut so the mounting bush has to be fairly long. Haven't found anyone who does all the values at a decent price yet.
The originals were made in the UK by Egen Electric Ltd, the component division of E.K.Cole Ltd, manufacturers of Ekco products.
Update Aug 2010. Maplin now have a range of pots made by Omeg which have a suitable thread length.
| | Original Value | Replacement | Maplin Code |
|---|
| RV1&2 | 500K log | 470K log | FW27E |
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| RV3&4 | 1M log | 1M log | FW28F |
|---|
| RV5 | 2M log | 2M2 log | FW29G |
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| RV6 | 250K log | 220K log | FW26D |
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| | | |
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The bodies are smaller (20mm) and the connecting lugs are for a pcb but they fit the front panel. That's about the nicest thing you could say about them because the construction and feel are disappointing. The 1M pots (the two with black inserts) have a ridiculous amount of side play in the spindles although the others are just about OK. They also need extra nuts (M10 x 0.75) which aren't generally available. Mouser or Conrad stock them or, for an easy life, you could just buy extra pots.
The track side of the control panel board faces the metalwork and, to avoid shorts, two strips of felt are glued to the aluminium. These are quite thick and it's important to space the board away from the panel to prevent mechanical stress.
In this case there's a lock washer between the first pot nut and the panel to make up the gap.
9) Close Relatives. The Selmer Stadium.
I was trying to find a schematic for the older Constellation 14 on the internet and only came across odd separate sheets for the Stadium. The Stadium is basically a Constellation 14 without the magic eye tremolo indicator and so I've put these sheets onto one drawing as a start. If anyone's got any Constellation 14 schematics I'd appreciate a copy. This is only a first attempt so there might be a few mistakes on it.
10) More soon